I have two issues. First of all, I purchased the seller vacation add on, downloaded and installed it, however, the settings that are supposed to appear in the sellers dashboard area are not there, the plugin does not seem to be working at all.

Also, for some reason, inside dokan > settings > selling options in the wp dashboard, I do not seem to have the withdrawal threshold option which is rather frustrating as it is an essential feature.

Upon further inspection, I have realised that the withdrawal threshold feature only comes with the latest version of the plugin. Unfortunately, I have made too many custom changes to my website to risk losing them by installing the latest version.

Therefore, is it possible for you to install this feature on my website as custom work and if so, how much will it cost?

You have to use our latest version to get rid from your issue. But as you have said you have customized your plugin and did not update the plugin yet then you have to add those feature by customization. We do not provide support for customization. You may require a developer to do this for you.